Recently, there have been developed precision machine tools such as vertical, horizontal or double-column precision machine tools or precision machine tools such as lathes, which are configured to machining, with high precision, molding dies for molding parts requiring a high machining precision, such as reflection mirrors and lenses of optical equipments.
A double-column machine tool is explained by way of example. The double-column type machine tool is a machine tool provided with a portal double-column in which a cross rail is horizontally extended between right and left columns mounted on opposed sides of a bed. A table is disposed between the right and left columns. A saddle holding a spindle head is disposed movably on the cross rail.
A conventional double-column type machine tool is generally of a large size, which is configured to machine large and long workpieces. However, a small-sized double-column type precision machine tool capable of carrying out a high-precision machining has been recently developed. Precision molding dies, which are used in producing parts related to precision parts and components of optical equipments, are machined by the small-sized double-column precision machine tool.
For example, in machining molding dies for use in molding optical components such as reflection mirrors and lenses, an ultra-precision accuracy in the order of sub-microns or nanometers is required in order to machine a die surface into a high-precision mirror surface. In the case of machining of a die having such a high-precision mirror surface is machined, a time required for completion of finish machining is inevitably long. Thus, it is necessary that a change in environmental temperature from the beginning to the end of machining does not affect a machining precision. Vertical and horizontal precision machine tools and vertical and horizontal lathes have the same necessity.
Generally in machine tools, since a bed and a column are thermally deformed by a heat generated by a motor and a guide surface and/or a variation in temperature, the column and the bed are provided with pipes through which cooling water flow, as countermeasures against the thermal deformation. As a conventional technique for cooling a column, a column cooling structure disclosed in, e.g., JP5-309536A and JP6-126564A can be taken by way of example.
In the case of a double-column type precision machine tool and a vertical precision machine tool, a cutting direction (Z axis) of a tool is vertical, which is the same as a column. When the column is thermally deformed under the influence of variation in temperature, the machining accuracy is quickly degraded and an ultra-precise machining such as a mirror surface becomes impossible.
Similarly to other machining machines, a precision machine tool is conventionally provided with a cooling unit configured to supply a cooling water to heat generation sources such as a motor for driving a axis and a guide surface. Although a part of the cooling water is circulated in the column, only a restricted portion of the column can be cooled. Thus, only a restrictive effect can be brought about in the case of ultra-precision machining.
In order to solve this problem, a precision machine tool is installed in a thermostatic room or a temperature inside a cover surrounding the precision machine tool is controlled. However, such a thermostatic room is insufficient to restrain a thermal displacement of a column, which is caused by a heat generated from the machine tool itself and/or an environmental temperature change, to such a degree that an ultra-precision machining can be carried out. Furthermore, there is a problem in that these equipments are extremely expensive.